The present invention concerns a compound film containing olefin, wherein at least one polyolefin layer has good adhesiveness to polymers containing polar radicals, such as nylon, polyester and polyolefins.
One of the well-known drawbacks of polyolefins, such as polyethylene and polypropylene, is that their adhesiveness to metals, and to other polymers, is poor. Many methods have been applied in attempts to improve the adhesiveness. As examples may be mentioned the treatment of the polyolefins' surface with acids, with a flame or a corona discharge, or the use of adhesive agents, such as ethylene-acrylic acid copolymers interposed between the polyolefin and the substrate in the case. The adhesiveness of polyolefins to polar polymers has also been improved by admixing to them polymers which possess good adhesiveness, or by copolymerizing polyethylene with monomers containing functional radicals.
Functional radicals improving the adhesiveness may also be obtained by grafting, to the polyolefin chains, various unsaturated polar compounds, such as acrylic acid, methacrylic acid and their derivatives (such as salts), and maleic anhydride.
Very high requirements are nowadays imposed on packaging products especially by the packaging of foodstuffs in plastic films. Therefore, the films intended to be used in packaging food may contain even a great number of separate layers, each one selected to endow the products with a certain characteristic, for instance impermeability to various gases and to water vapor, permeability to certain gases, joinability by seaming etc. In frequent instances a given layer is only required to enable layers with poor adhesiveness, such as polyolefin layers for instance, to be made adherent to other layers containing polar polymers.
The methods employed with a view to improving the adhesiveness of polyolefin layers are encumbered by many drawbacks. For instance, grafting of maleic anhydride or of acrylic acid to polyolefin is risky from the viewpoint of foodstuff applications because said substances can be considered toxic. Furthermore, for instance the adding of maleic anhydride in connection with film extruding operations is difficult and implies the use of solvents.